This invention provides a sealing spacer for spacing apart two window panes to form a window assembly. The spacer has an elongated, flexible strip having opposed edge surfaces and opposed side surfaces. The spacer also has a fiber reinforced polymer layer over the elongated, flexible strip. The opposed edge surfaces undulate with crests and troughs. The spacer has an activatable sealant for directly sealingly securing the flexible strip to each of the two window panes. The activatable sealant is on each of the opposed side surfaces of the fiber reinforced polymer. The invention also provides methods for making the spacer and window assembly.